What do those #-signs mean? For example,

data Word = W# Word# deriving (Eq, Ord)

I tried to google it but I found nothing.

  • On the theme of learning how to fish: Here is a list of all built in operators in haskell. For operators that are just a library function and not built in, you can try Hoogle and Hayoo. The info tab for haskell on StackOverflow has a nice summary of common questions and useful resources.
    – Hjulle
    Commented Nov 6, 2015 at 11:28

1 Answer 1


Those are unboxed types, see the corresponding section of the haskell docs

They are mainly used for performance or interop reasons(along with the primitive operations that work on them, for example +#)

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